1. Field of the Invention
The present invention relates to a gas sensor having a stacked-type gas detecting element in which a plurality of ceramic layers are stacked, as well as a method for manufacturing the same.
2. Description of the Related Art
Conventionally, a plate-like gas detecting element is known which extends in a longitudinal direction. A plurality of ceramic layers are stacked to form the plate-like gas detecting element, and a detecting portion is formed at a leading end side thereof. Such gas detecting elements are disclosed, for example, in JP-A-61-134655, JP-A-2001-242129, JP-A-2001-311714 and JP-A-2002-107335. Through holes penetrating the ceramic layers are provided in the gas detecting element. In each of these through holes, a conductor is provided for electrically connecting a lead portion extending from a sensing electrode disposed in the interior of the gas detecting element and an electrode pad disposed on an outer surface of the gas detecting element.
3. Problems to be Solved by the Invention
The present inventors invented a gas detecting element 900 shown in FIG. 9. Specifically, a solid electrolyte layer 913 has a first through hole 913h penetrating between the first surface 913a and the second surface 913b of the solid electrolyte layer 913, and the second surface 913b side of the first through hole 913h is closed by a first insulating layer 911. A first through-hole conductor 921 is formed on the inner peripheral surface of the first through hole 913h so as to connect a first connecting layer 923 formed on the first surface 913a of the solid electrolyte layer 913 and a lead portion 931 formed on the second surface 913b of the solid electrolyte layer 913.
In addition, a second through hole 915h is formed in a second insulating layer 915, and a second through-hole conductor 925 is formed on a portion (left side in FIG. 9) of the inner peripheral surface of this second through hole 915h. Further, a second connecting layer 927, which is connected to the second through-hole conductor 925 and is overlappingly connected to the first connecting layer 923, is formed on the first connecting layer 923. Furthermore, a filled through-hole conductor 929, which is connected to the second connecting layer 927, is filled and formed in the first through hole 913h (on the inner side of the first through-hole conductor 921) of the solid electrolyte layer 913.
Such a gas detecting element 900 can be formed as follows: Namely, an unsintered solid electrolyte layer is prepared which forms the solid electrolyte layer 913 after sintering. Then, a first unsintered through-hole conductor, which forms the first through-hole conductor 921 after sintering, is formed on the inner peripheral surface of its first through hole by printing a conductor paste. In addition, a first unsintered connecting layer, which forms the first connecting layer 923 after sintering, is formed on the first surface of this unsintered solid electrolyte layer, and an unsintered lead portion, which forms the lead portion 931 after sintering, is formed on the second surface of this unsintered solid electrolyte layer.
Subsequently, the unsintered solid electrolyte layer is stacked on the first unsintered insulating layer which forms the first insulating layer 911 after sintering. Furthermore, a second unsintered insulating layer, which forms the second insulating layer 915 after sintering, is formed on this stacked body.
Next, a second unsintered through-hole conductor which forms the second through-hole conductor 925 after sintering, a second unsintered connecting layer which forms the second connecting layer 927 after sintering, and an unsintered filled through-hole conductor which forms the filled through-hole conductor 929 after sintering, are formed by printing the conductor paste.
Then, upon sintering, the stacked body of the unsintered ceramic, the gas detecting element 900 is formed.
However, with such a gas detecting element 900, there are cases where a crack occurs in the filled through-hole conductor 929 itself or at a connecting layer between the filled through-hole conductor 929 and the other conductor 931 during sintering. Conceivably, the reason is that since the amount of shrinkage upon sintering differs among the unsintered solid electrolyte layer, the unsintered insulating layer (first unsintered insulating layer) and the unsintered conductor, a large stress is applied to the filled through-hole conductor 929 having a relatively large volume and to a connecting layer between the same and the other conductor 931. If such a crack occurs, the electrical connection reliability may suffer.